Download IR Spectroscopy

Determining the Structure of an
Organic Compound
 The analysis of the outcome of a reaction requires
that we know the full structure of the products as well
as the reactants
 In the 19th and early 20th centuries, structures were
determined by synthesis and chemical degradation
that related compounds to each other.
 Physical methods now permit structures to be
determined directly. We will examine:


infrared (IR) spectroscopy
nuclear magnetic resonance spectroscopy (NMR)
1
Principles of Spectroscopy
The relationship between energy of light E, and its
frequency,  is given by the equation:
E=h
The equation says that there is a direct relationship
between the frequency of light and its energy: the
higher the frequency, the higher the energy.
The proportionality constant between energy and
frequency is known as Plank’s constant, h.
2
Principles of Spectroscopy
Also,  = c / , where c is the speed of light and  is
the wavelength of light.
Therefore, E = h c / 
This equation tells us that the shorter the wavelength
of light, the higher its energy.
3
Principles of Spectroscopy
Molecules can exist at various energy levels.
For example,
The bonds in a given molecule may stretch, bend, or rotate.
electrons may move from one orbital to another.
These processes are quantized; that is, bonds may stretch,
bend or rotate only with certain frequencies, and electrons may
jump between orbitals with well defined energy differences.
It is these energy differences that we measure by various types
of spectroscopy.
4
Spectroscopy of the Electromagnetic Spectrum
 Radiant energy is proportional to its frequency
(cycles/s = Hz) as a wave (Amplitude is its height)
 Different types are classified by frequency or
wavelength ranges
5
Infrared Spectroscopy of Organic
Molecules
 All covalent compounds absorb various frequencies of
electromagnetic radiation in the infrared region of the
electromagnetic spectrum
 IR energy in a spectrum is usually measured as wave number
(cm-1), the inverse of wavelength and proportional to frequency.
 The main reason to use wavenumbers as units is that they are
directly proportional to energy (a higher wavenumber
corresponds to higher energy).
 In terms of wavenumbers, the IR extends from 4000 to 400cm-1.
6
The infrared absorption process
 Molecules are excited to a higher energy state when they
absorb infrared (IR) radiation.
 The absorption of IR radiation is a quantized process.
 The absorption of IR radiation corresponds to energy changes in
the order of 8 – 40 KJ/mol (1.9 – 9.5 KCal/mol).
 Radiation in this energy range corresponds to the stretching and
bending vibrational frequencies of the bonds in most covalent
molecules.
7
The infrared absorption process
 When a match occurs between the frequencies of the IR
radiation and the natural vibrational frequencies of the molecule,
energy is absorbed by the molecule.
 The absorbed energy serves to increase the amplitude of the
vibrational motions of the bonds in the molecule.
8
The infrared absorption process
 All the bonds in a molecule are not
capable of absorbing IR energy, even,
if the frequency of the radiation exactly
matches that of the bond motion.
 Only those bonds that have a dipole
moment that changes as a function of
time are capable of absorbing the IR
radiation.
 Thus, a symmetric bond which has identical or nearly identical groups on
each end will not absorb in the infrared.
9
The infrared absorption process
 A symmetric bond which has identical or nearly identical groups
on each end will not absorb in the infrared.
10
Infrared Energy Modes
 Stretching and bending vibrational motions (modes) that are infrared
active give rise to absorptions.
 Asymmetric stretching vibrations occur at higher frequencies than
symmetric stretching vibrations.
 Stretching vibrations occur at higher frequencies than bending
vibrations.
 Bending vibrations :
1) In-plane
a) Scissoring
b) Rocking
2) Out-of-plane
a) Wagging
b) Twisting
11
Gas phase IR spectrum of formaldehyde
12
Overtone, combination and difference bands, and Fermi
resonance

Apart from fundamental absorptions due to excitation from ground state to the lowest-energy excited state,
the spectrum is usually complicated by the presence of weak overtone, combination, and difference bands.

Overtone Bands
Excitation from ground state to higher energy states, which corresponds integral multiples of the frequency
of the fundamental (). Weak overtone bands could occur at 2, 3, ….

Combination bands
Two vibrational frequencies (1 and 2) in a molecule couple to give rise to a new infrared active frequency.
This band is the sum of the two interacting bands (combination = 1 + 2).

Difference bands
A new infrared active frequency arising due to the difference between the two interacting bands (difference =
1 - 2).

Fermi resonance
When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called
Fermi resonance. Fermi resonance is often observed in carbonyl compounds.

These additional weak bands can be calculated directly by multiplication, addition, and subtraction
processes.
13
Effect of rotational frequencies
 Rotational frequencies of the whole molecule are not infrared
active
 However, they often couple with the stretching and bending
vibrations in the molecule to give additional fine structures to
these vibrations.
 One of the reasons a band is broad rather than sharp in the IR
spectrum is rotational coupling.
14
Bond Properties
 The natural frequency of vibration of a bond is given by the
equation:
where µ = m1m2/(m1+m2) (termed the 'reduced mass'), and c
is the velocity of light.
K is a constant that varies from one bond to another. Force
constants for triple bonds are three times those of single
bonds.
 Stronger bonds have a large force constant K and vibrate at
higher frequencies
 Bonds between atoms of higher masses vibrate at lower
frequencies than bonds between lighter atoms.
15
Bond Properties
 Hybridization affects the force constant K.
Bonds are stronger in the order sp > sp2 >
sp3.
 Resonance affects the strength and length of
a bond and hence its force constant.
16
Interpreting Infrared Spectra
 Most functional groups absorb at about the
same energy and intensity independent of the
molecule they are in.
 Characteristic higher energy IR absorptions
can be used to confirm the presence of a
functional group in a molecule.
 IR spectrum has lower energy region
characteristic of a molecule as a whole
(“fingerprint” region).
17
Regions of the Infrared Spectrum
 4000-2500 cm-1 N-H, C-
 2000-1500 cm-1 double
H, O-H (stretching)
 3300-3600 N-H, O-H
 3000 C-H
 2500-2000 cm-1 CC
and C  N (stretching)
bonds (stretching)
 C=O 1680-1750
 C=C 1640-1680 cm-1
 Below 1500 cm-1
“fingerprint” region
18
Appearance of IR Spectrum
19
Usefulness of infrared absorption spectroscopy:
Five C4H8O isomers
20
Carbonyl Compounds
Aldehydes and Ketones
For simple aldehydes and ketones, the stretching vibration
of the carbonyl group gives rise to a strong and distinctive
infrared absorption at 1710 to 1740 cm-1.
Since alkyl substituents stabilize the carbocation character of
the ionic contributer, ketone carbonyls have slightly lower
stretching frequencies, 1715 ± 7 cm-1, compared with
aldehydes, 1730 ± 7 cm-1.
21
Carbonyl Compounds - Aldehydes and
Ketones
Three factors are known to perturb the carbonyl stretching
frequency:
1) Conjugation with a double bond or benzene ring lowers
the stretching frequency.
22
Carbonyl Compounds
Aldehydes and Ketones
2) Incorporation of the carbonyl group in a small ring (5,4, or 3)\
Raises the stretching frequency.
23
Carbonyl Compounds
Aldehydes and Ketones
3) Changing an alkyl substituent of a ketone for an electron
releasing or withdrawing group.
24
Carbonyl Compounds
Aldehydes and Ketones
25
Carbonyl Compounds
Aldehydes and Ketones
26
Carbonyl Compounds
Aldehydes and Ketones
27
Carbonyl Compounds
Aldehydes and Ketones
28
Carboxylic Acids
The carboxyl group is associated with two characteristic infrared stretching
absorptions which change markedly with hydrogen bonding.
Carboxylic acids exist predominantly as hydrogen bonded dimers in condensed
phases.
The O-H stretching absorption for such dimers is very strong and broad,
extending from 2500 to 3300 cm-1.
This absorption overlaps the sharper C-H stretching peaks, which may be seen
extending beyond the O-H envelope at 2990, 2950 and 2870 cm-1.
The smaller peaks protruding near 2655 and 2560 are characteristic of the
dimer.
The carbonyl stretching frequency of the dimer is found near 1710 cm-1, but is
increased by 25 cm-1 or more in the monomeric state.
29
Carboxylic Acids
30
Carboxylic Acids
31
Carboxylic Acids
32
Carboxylic Acid Derivatives
Acyl Halides
Acyl Halides (RCOX)
The very reactive acyl halides absorb at frequencies significantly higher than
ketones.
C=O stretch:
X = F, 1860 ± 20 cm-1; X = Cl, 1800 ± 15; X = Br,1800 ± 15
33
Carboxylic Acid Derivatives
Acid Anhydride
Acid Anhydride, (RCO)2O : The very reactive anhydrides absorb at
frequencies significantly higher than ketones.
C=O stretch: (2 bands separated by 60  30 cm-1) acyclic, 1750 & 1820 cm-1;
6-membered ring, 1750 & 1820; 5-membered ring, 1785 & 1865.
34
Carboxylic Acid Derivatives
Esters & Lactones
Esters & Lactones (RCOOR'):
C=O stretch: Esters, 1740  10 cm-1; 6 membered lactone, 1740  10 cm-1;
5 membered lactone, 1765  10 cm-1; 4 membered lactone, 1840  5 cm-1
35
Carboxylic Acid Derivatives
Amides & Lactams
Amides & Lactams (RCONR2): The relatively unreactive amides absorb at lower
frequencies. C=O bands : 1° & 2°-amides, 1510 to 1700 cm-1 (2 bands)
3°-amides, 1650± 15 (one band); 6-membered lactams, 1670 ± 10 (one band)
5-membered lactams, 1700 ± 15 ; 4-membered lactams, 1745 ± 15.
36
Alcohols and Phenols
The O-H stretching absorption of the hydroxyl group is sensitive to hydrogen
bonding.
In the gas phase and in dilute CCl4 solution (0.01 M) small to moderate sized
alcohols exhibit a sharp absorption in the 3620 to 3670 cm-1 region.
In more concentrated solution, or as a pure liquid, hydrogen bonding of the hydroxyl
groups to each other occurs, and this lowers and broadens the stretching
frequencies of the participating O-H bonds.
37
Amines
Primary Amine (1°): Primary aliphatic amines display two well-defined peaks due to
asymmetric (higher frequency) and symmetric N-H stretching, separated by 80 to 100
cm-1. Strong in-plane NH2 scissoring absorptions at 1550 to 1650 cm-1, and out-of-plane
wagging at 650 to 900 cm-1 (usually broad) are characteristic of 1°-amines.
38
Amines
Secondary Amine (2°): Secondary amines exhibit only one absorption near 3420 cm-1.
Hydrogen bonding in concentrated liquids shifts these absorptions to lower frequencies
by about 100 cm-1.
39
Amines
Tertiary Amine (3°): No N-H absorptions. The C-N absorptions are found in the same
range, 1200 to 1350 cm-1 (aromatic) and 1000 to 1250 cm-1 (aliphatic) as for 1°-amines.
40
Arenes
In addition to the stretching vibrations at 3000  70 cm-1, several sharp, weaker
absorptions in the 950 to 1250 range due to in-plane C-H bending vibrations also
occur.. These are not diagnostically useful, except for indicating a substituted benzene
ring. Weak overtone and combination tone absorptions are found in the 1600-2000
region
41
Arenes
Ortho substituted dialkyl arene
42
Arenes
Meta substituted dialkyl arene
43
Arenes
Para substituted dialkyl arene
44